1. Field of the Invention
The present invention relates to an electrode structure for a polymer electrolyte fuel cell and a method for manufacturing the same.
2. Description of the Related Art
Petroleum resources are going to be exhausted, and at the same time environmental issues such as global warming due to the consumption of fossil fuels are becoming more critical. Thus, as a clean power source for an electric motor that is not associated with the generation of carbon dioxide, a fuel cell has attracted attention, has been developed extensively, and has become commercially practical in some applications. When the fuel cell is mounted on a vehicle and the like, a polymer electrolyte fuel cell using a polymer electrolyte membrane is suitably used because high voltage and a large current are readily obtained.
There is known an electrode structure for use in the polymer electrolyte fuel cell comprising a pair of electrode catalyst layers which is formed by supporting a catalyst such as platinum on a catalyst carrier such as carbon black and being integrated by an ion conducting polymer binder, wherein an ion-conducting polymer electrolyte membrane is sandwiched between the both electrode catalyst layers and a diffusion layer is laminated on each of the electrode catalyst layers (Refer to, for example, Japanese Patent Laid-Open No. 2000-223136). The electrode structure can compose a polymer electrolyte fuel cell by further laminating a separator, which also serves as a gas channel, on each diffusion layer.
In the polymer electrolyte fuel cell, one of the electrode catalyst layers is used as a fuel electrode, into which a reducing gas such as hydrogen or methanol is introduced through the diffusion layer, and the other electrode catalyst layer is used as an oxygen electrode, into which an oxidizing gas such as air or oxygen is introduced through the diffusion layer. In this way, at the fuel electrode side, the catalyst contained in the electrode catalyst layer acts to produce protons from the reducing gas, and the protons move through the polymer electrolyte membrane to the electrode catalyst layer at the oxygen electrode side. Further, the protons react with the oxidizing gas introduced into the oxygen electrode side to produce water in the electrode catalyst layer at the oxygen electrode by the action of the catalyst contained in the electrode catalyst layer. Consequently, the polymer electrolyte fuel cell can provide current by connecting the fuel electrode to the oxygen electrode with a lead wire.
Conventionally, in the electrode structure, perfluoroalkylenesulfonic acid polymer compound (e.g., Nafion (trade name) made by E.I. du Pont de Nemours and Company) has been widely utilized as the polymer electrolyte membrane. The perfluoroalkylenesulfonic acid polymer compound has excellent proton conducting properties as it is sulfonated and also has chemical resistance as a fluoropolymer, but has a problem that it is very expensive.
Therefore, it has been studied to compose an electrode structure for a polymer electrolyte fuel cell using a less expensive ion-conducting material as an alternative to perfluoroalkylenesulfonic acid polymer compound. For example, a sulfonated polyarylene-based polymer is known as the less expensive ion-conducting material.
However, the electrode structure using the polymer electrolyte membrane made of the sulfonated polyarylene-based polymer has the problem of degradation of the polymer electrolyte membrane by the heat during operation, when composing a fuel cell, causing to mix the gases introduced into the fuel electrode and oxygen electrode sides and to develop cross-leak in which the poles are short-circuited.
Conventionally, the electrode structure has been produced, for example, as described below. First, a polymer electrolyte membrane is formed by a casting method from a solution prepared by dissolving the sulfonated polyarylene-based polymer in a solvent such as N-methylpyrrolidone.
Catalyst particles in which platinum particles are supported on carbon particles are dispersed in the polymer electrolyte solution to prepare a catalyst paste containing the catalyst particles and the polymer electrolyte. The catalyst paste is coated on a sheet-like support such as a polyethylene terephthalate film and dried to form an electrode catalyst layer.
Then, the both sides of the polymer electrolyte membrane are sandwiched between the electrode catalyst layers and maintained at the temperature ranging from 80 to 160° C. After the polymer electrolyte membrane and the polymer electrolyte contained in the electrode catalyst layers are softened, they are maintained under a pressure in the range of 1 to 10 MPa for 1 to 60 minutes. As a result, the electrode catalyst layers are transferred to the polymer electrolyte membrane from the polyethylene terephthalate film to be joined to the polymer electrolyte membrane by thermocompression bonding.
Then, the electrode catalyst layers at the both sides are sandwiched between diffusion layers and subjected to hot press. Thereby, the diffusion layers are joined to each of the electrode catalyst layers to form an electrode structure.
However, the electrode structure produced by the conventional production method has a problem that it has large change in dimensions.